Charmsync Leadership election

charm-helpers-hooks.yaml

@@ -1,4 +1,4 @@
-branch: lp:charm-helpers
+branch: lp:~le-charmers/charm-helpers/leadership-election
 destination: hooks/charmhelpers
 include:
     - core

hooks/charmhelpers/contrib/hahelpers/cluster.py

@@ -44,6 +44,7 @@ from charmhelpers.core.hookenv import (
     ERROR,
     WARNING,
     unit_get,
+    is_leader as juju_is_leader
 )
 from charmhelpers.core.decorators import (
     retry_on_exception,
@@ -66,12 +67,18 @@ def is_elected_leader(resource):
     Returns True if the charm executing this is the elected cluster leader.
 
     It relies on two mechanisms to determine leadership:
-        1. If the charm is part of a corosync cluster, call corosync to
+        1. If juju is sufficiently new and leadership election is supported,
+        the is_leader command will be used.
+        2. If the charm is part of a corosync cluster, call corosync to
         determine leadership.
-        2. If the charm is not part of a corosync cluster, the leader is
+        3. If the charm is not part of a corosync cluster, the leader is
         determined as being "the alive unit with the lowest unit numer". In
         other words, the oldest surviving unit.
     """
+    try:
+        return juju_is_leader()
+    except NotImplementedError:
+        pass
     if is_clustered():
         if not is_crm_leader(resource):
             log('Deferring action to CRM leader.', level=INFO)

hooks/charmhelpers/contrib/peerstorage/__init__.py

@@ -19,9 +19,11 @@ from charmhelpers.core.hookenv import relation_id as current_relation_id
 from charmhelpers.core.hookenv import (
     is_relation_made,
     relation_ids,
-    relation_get,
+    relation_get as _relation_get,
     local_unit,
-    relation_set,
+    relation_set as _relation_set,
+    leader_get,
+    leader_set,
 )
 
 
@@ -54,6 +56,26 @@ def some_hook():
 """
 
 
+def relation_set(relation_settings=None, relation_id=None, **kwargs):
+    try:
+        if relation_id and relation_id in relation_ids('cluster'):
+            return leader_set(settings=relation_settings, **kwargs)
+        else:
+            raise NotImplementedError
+    except NotImplementedError:
+        return _relation_set(relation_settings=relation_settings, **kwargs)
+
+
+def relation_get(attribute=None, rid=None, unit=None):
+    try:
+        if relation_id and relation_id in relation_ids('cluster'):
+            return leader_get(attribute)
+        else:
+            raise NotImplementedError
+    except NotImplementedError:
+        return _relation_get(attribute=attribute, rid=rid, unit=unit)
+
+
 def peer_retrieve(key, relation_name='cluster'):
     """Retrieve a named key from peer relation `relation_name`."""
     cluster_rels = relation_ids(relation_name)

hooks/charmhelpers/core/hookenv.py

@@ -566,3 +566,62 @@ class Hooks(object):
 def charm_dir():
     """Return the root directory of the current charm"""
     return os.environ.get('CHARM_DIR')
+
+
+def translate_exc(from_exc, to_exc):
+    def inner_translate_exc1(f):
+        def inner_translate_exc2(*args, **kwargs):
+            try:
+                return f(*args, **kwargs)
+            except from_exc:
+                raise to_exc
+
+        return inner_translate_exc2
+
+    return inner_translate_exc1
+
+
+@translate_exc(from_exc=CalledProcessError, to_exc=NotImplementedError)
+def is_leader():
+    """Does the current unit hold the juju leadership
+
+    Uses juju to determine whether the current unit is the leader of its peers
+    """
+    try:
+        leader = json.loads(
+            subprocess.check_output(['is-leader', '--format=json'])
+        )
+        return (leader is True)
+    except ValueError:
+        raise NotImplementedError
+
+
+@cached
+@translate_exc(from_exc=CalledProcessError, to_exc=NotImplementedError)
+def leader_get(attribute=None):
+    """Juju leader get value(s)"""
+    cmd = ['leader-get', '--format=json'] + [attribute or '-']
+    try:
+        return json.loads(subprocess.check_output(cmd).decode('UTF-8'))
+    except ValueError:
+        return None
+    except CalledProcessError as e:
+        if e.returncode == 2:
+            return None
+
+        raise
+
+
+@translate_exc(from_exc=CalledProcessError, to_exc=NotImplementedError)
+def leader_set(settings=None, **kwargs):
+    """Juju leader set value(s)"""
+    cmd = ['leader-set']
+    settings = settings or {}
+    settings.update(kwargs)
+    for k, v in settings.iteritems():
+        if v is None:
+            cmd.append('{}='.format(k))
+        else:
+            cmd.append('{}={}'.format(k, v))
+
+    subprocess.check_call(cmd)